The selective destruction of an individual cell is often desirable in a variety of clinical settings. A multitude of signal transduction pathways in the cell are linked to its death and survival, and delivery of a limiting and/or crucial component of the pathway can be productive in terms of its destruction. A classic example of such a signal transduction pathway is apoptosis, and a variety of elements of apoptotic pathways would be useful to target a cell for death. Apoptosis, or programmed cell death, is a fundamental process controlling normal tissue homeostasis by regulating a balance between cell proliferation and death (Vaux et al., 1994; Jacobson et al., 1997).
The serine protease granzyme B (GrB) (Lobe et al., 1986; Schmid and Weissman, 1987; Trapani et al., 1988) is integrally involved in apoptotic cell death induced in target cells upon their exposure to the contents of lysosome-like cytoplasmic granules (or cytolytic granules) found in cytotoxic T-lymphocytes (CTL) and natural killer (NK) cells (Henkart, 1985; Young and Cohn, 1986; Smyth and Trapani, 1995). Cytotoxic lymphocyte granules contain perforin, a pore-forming protein, and a family of serine proteases, termed granzymes (Table 1). Perforin has some structural and functional resemblance to the complement proteins C6, C7, C8 and C9, members of complement membrane attack complex (Shinkai et al., 1988). In lymphocyte-mediated cytolysis, perforin is inserted into the target cell membranes and appears to polymerize to form pores (Podack, 1992; Yagita et al., 1992), which mediates access of granzyme B to the target cell cytoplasm. Once inside, granzyme B induces apoptosis by directly activating caspases and inducing rapid DNA fragmentation (Shi et al., 1992).
TABLE 1GRANZYMES (LYMPHOCYTE SERINE PROTEASES)EnzymeNamesSpeciesOther NamesActivityAMouseHanukah factor, MTSP, SE-1, CTLA-3TryptaseRatRNKP-2, fragmentin 1HumanHanukah factor, HTSP-1, granzyme 1BMouseCCP-1, CTLA-1Asp-aseRatFragmentin 2, RNKP-1HumanHLP, granzyme 2, HSE26.1, CSPBCMouseCCP-2UnknownRatRNKP-4DMouseCCP-5UnknownEMouseCCP-3, MCSP2UnknownFMouseCCP-4, MCSP3UnknownGMouseMCSP1UnknownHHumanCCP-X, CSP-CChymaseIRatGLP I and IIUnknownJRatRNKP-5UnknownKRatTryptase 2, fragmentin 3TryptaseHumanGranzyme 3TryptaseMRatRNK-Met-1Met-aseHumanMet-ase
The granzymes are structurally related, but have diverse substrate preference. Through its unique ability to cleave after aspartate residues, granzyme B can cleave many procaspases in vitro, and it has been an important tool in analyzing the maturation of caspase-3 (Darmon et al., 1995; Quan et al., 1996; Martin et al., 1996), caspase-7 (Chinnaiyan et al., 1996; Gu et al., 1996; Femandes-Alnemri et al., 1995), caspase-6 (Orth et al., 1996; Ferrandes-Alnemri et al., 1995), caspase-8 (Muzio et al., 1996), caspase-9 (Duan et al., 1996), and caspase-10a/b (Fernandes-Alnemri et al., 1996; Vincenz and Dixit, 1997). Furthermore, it is highly toxic to target cells (Shi et al., 1992). It has been assumed until now that granzyme B kills cells by direct caspase activated, supplemented under certain circumstances by direct damage to downstream caspase substrates (Andrade et al., 1998). Having gained access to the cytosol, granzyme B is rapidly translocated to the nucleus (Jans et al., 1996; Trapani et al., 1996) and can cleave poly (ADP-ribose) polymerase and nuclear matrix antigen, sometimes using different cleavage sites than those preferred by caspases (Andrade et al., 1998). Although many procaspases are efficiently cleaved in vitro, granzyme B-induced caspase activation occurs in a hierarchical manner in intact cells, commencing at the level of executioner caspases such as caspase-3, followed by caspase-7 (Yang et al., 1998). This is in contrast to FasL-mediated killing, which relies on a membrane signal generated through apical caspases such as caspase-8 (Muzio et al., 1996; Sarin et al., 1997). In addition, some studies showed that granzyme B can also induce death through a caspase-independent mechanism that involves direct damage to nonnuclear structures, although the key substrates in this pathway have yet to be elucidated (Sarin et al., 1997; Trapani et al., 1998; Heibein et al., 1999; Beresford et al., 1999).
Studies by Froelich and co-workers suggest that GrB is internalized by receptor-mediated endocytosis, and that the role of perforin is to mediate release of granzyme B from endocytic vesicles. In fact, perforin can be replaced by other vesicle-disrupting factors such as those produced by adenovirus (Froelich et al., 1996; Pinkoski et al., 1998; Browne et al., 1999).
Granzymes in general are highly homologous, with 38–67% homology to GrB (Haddad et al., 1991), and they contain the catalytic triad (His-57, Asp-102, and Ser-195) of trypsin family serine proteases. Other features include the mature, N-terminal Ile-Ile-Gly-Gly sequence, three or four disulfide bridges, and a conserved motif (PHSRPYMA), which also appears in neutrophil cathepsin G and mast cell chymases. The carbohydrate moieties of granzymes are Asn-linked (Griffiths and Isaaz, 1993). The granzyme mRNA transcripts are translated as pre-pro-proteases. The pre- or leader sequence is cleaved by signal peptidase at the endoplasmic reticulum. When the propeptides are removed, the inactive progranzymes (zymogens) become active proteases. The granzyme propeptides sequences start after the leader peptide and end before the N-terminal Ile needed for the protease to fold into a catalytic conformation (Kam et al., 2000).
Among the various apoptotic factors identified so far, members of the Bcl-2 family represent some of the most well-defined regulators of this death pathway. Some members of the Bcl-2 family, including Bcl-2, Bcl-XL, Ced-9, Bcl-w and so forth, promote cell survival, while other members including Bax, Bcl-Xs, Bad, Bak, Bid, Bik and Bim have been shown to potentiate apoptosis (Adams and Cory, 1998). A number of diverse hypotheses have been proposed so far regarding the possible biological functions of the Bcl-2 family members. These include dimer formation (Oltvai et al., 1993), protease activation (Chinnaiyan et al., 1996), mitochondrial membrane depolarization (6), generation of reactive oxygen intermediates (Hockenbery et al., 1993), regulation of calcium flux (Lam et al., 1994; Huiling et al., 1997), and pore formation (Antonsson et al., 1997; Marzo et al., 1998).
Bax, a 21 kDa death-promoting member of the Bcl-2 family, was first identified as a protein that co-immunoprecipated with Bcl-2 from different cell lines (Oltvai et al., 1993). Overexpression of Bax accelerates cell death in response to a wide range of cytotoxic results. Determination of the amino acid sequence of the Bax protein showed it to be highly homologous to Bcl-2. The Bax gene consists of six exons and produces alternative transcripts, the predominant form of which encodes a 1.0 kb mRNA and is designated Baxα. Like Bcl-2 and several other members of the Bcl-2 family, the Bax protein has highly conserved regions, BH1, BH2 and BH3 domains, and hydropathy analysis of the sequences of these proteins indicates the presence of a hydrophobic transmembrane segment at their C-terminal ends (Oltvai et al., 1993).
Bax is widely expressed without any apparent tissue specificity. However, on the induction of apoptosis, Bax translocates into mitochondria, resulting in mitochondria dysfinction and release of cytochrome c, which subsequently activates caspase pathways (Hsu and Youle, 1997; Wolter et al., 1997; Gross et al., 1998). This translocation process is rapid and occurs at an early stage of apoptosis (Wolter et al., 1997). Selective overexpression of Bax in human ovarian cancer through adenoviral gene transfer resulted in significant tumor cell kill in vivo (Tai et al., 1999). Overexpression of the Bax gene by a binary adenovirus system in cultured cell lines from human lung carcinoma results in caspase activation, apoptosis induction, and cell growth suppression. Moreover, intratumoral injection of adenovirus vector expressing the Bax gene suppressed growth of human lung cancer xenografts established in nude mice (Kagawa et al., 2000; Kagawa et al., 2000).
WO 99/45128 and Aqeilan et al. (1999) are directed to chimeric proteins having cell-targeting specificity and apoptosis-inducing activities, particularly the recombinant chimeric protein IL-2-Bax, which specifically targets IL2 receptor-expressing cells and induces cell-specific apoptosis.
WO 99/49059 relates to a chimeric toxin comprised of gonadotropin releasing hormone (GnRH) and Pseudomonas exotoxin A (PE) to detect a tumor-associated epitope expressed by human adenocarcinoma.
WO 97/46259 concerns targeted chimeric toxins comprising cell targeting moieties and cell killing moieties directed to neoplastic cells. In a specific example, the chimeric toxin comprises gonadotropin releasing hormone homologs and Pseudomonas Exotoxin A.
WO 97/22364 addresses targeted treatment of allergy responses, whereby a chimeric cytotoxin Fc2′-3-PE40 is directed to targeted elimination of cells expressing the FcεRI receptor.
While some chimeric protein compositions have been described, other methods and compositions are needed for improved therapies involving the killing of cells.